Process for reducing the formation of polymer attendant the pyrolysis of tetrafluoroethylene

ABSTRACT

THE PYROLYSIS OF TETRAFLUOROETHYLENE TO HIGHER MOLECULAR WEIGHT PERFLUOROCARBON COMPOUNDS, E.G., HEXAFLUOROPROPYLENE, IS ACCOMPANIED BY POLYMER FORMATION WHICH, ACCORDING TO THE PRESENT INVENTION, IS SUPPRESSED BY INTIMATELY CONTACTING THE PYROLYZATE BEFORE IT COOLS SUFFICIENTLY FOR POLYMERIZATION TO OCCUR WITH SUCH COMPOUNDS AS HC1, HF, KF OR NAF.

United States Patent PROCESS FOR REDUCING THE FORMATION OF POLYMERATTENDANT THE PYROLYSIS F TETRAFLUOROETHYLENE Maurice Joseph Couture,Parkersburg, W. Va., and Hans M. B. Gerner and Hugh H. Gibbs,Wilmington, Del., assignors to E. I. du Pout de Nemours and Company,Wilmington, Del. No Drawing. Filed Dec. 14, 1967, Ser. No. 690,398

Int. Cl. C07c 17/26, 17/28, 23/06 US. Cl. 260-648 6 Claims ABSTRACT OFTHE DISCLOSURE The pyrolysis of tetrafluoroethylene to higher molecularweight per-fluorocarbon compounds, e.g., hexafluoropropylene, isaccompanied by polymer formation which, according to the presentinvention, is suppressed by intimately contacting the pyrolyzate beforeit cools sufiiciently for polymerization to occur with such compounds asHCl, HF, KF or NaF.

This invention relates to the pyrolysis of tetrafluoroethylene and moreparticularly, to a process for suppressing the formation of polymerattending this pyrolysis.

The pyrolysis of tetrafluoroethylene to useful higher molecular weightfluorocarbon compounds, such as hexafiuoropropylene andoctafluorocyclobutane, is disclosed in US. Pat. No. 2,404,374 (Reissue23,425) to Harmon and US. Pat. No. 2,436,142 to Harmon. An improvedmethod for pyrolyzing tetrafluoroethylene to hexafiuoropropylene isdisclosed in US. Pat. No. 2,758,138 to Nelson, wherein the pyrolysis isconducted at 750 to 900 C., to 5,000 grams per liter hour, and 25 to 200mm. Hg. A process which is conducted at higher pressures namely, 0.2 to65 psi, is disclosed in US. Pat. No. 2,970,176 to Ten Eyck et al.,wherein the tetrafluoroethylene starting material contains at least 5weight percent of dimer and other high boilers.

Still another method for obtaining hexafluoropropylene by pyrolysis isdisclosed in U.S. Pat. No. 3,306,940 to Halliwell, wherein instead oftetrafluoroethylene being the starting material, the starting materialis chlorodifluoromethane, and the pyrolysis is conducted to a conversionof from 86 to 94 percent of the chlorodifluoromethane to produce apyrolyzate containing both tetrafluoroethylene and hexafluoropropylene.While this cosynthesis process has the advantage of producing a highyield of useful products relative to the amount of waste productsformed, it also has the disadvantage of low productivity ofhexafluoropropylene relative to the capacity of the equipment beingused. In other words, if hexafluoropropylene is the desired product,then higher productivity of this product is obtained from a givenfurnace when the starting material is tetrafluoroethylene rather thanchlorodifluoromethane.

Unfortunately, however, when tetrafluoroethylene has been used as thestarting material, such as in the above described patent disclosedprocesses, the formation of the desired higher molecular weightperfluorocarbon compounds, e.g., hexafiuoropropylene and/oroctafluorocyclobutane, has been accompanied by the formation of solidpolymer which builds up in the recovery system following the pyrolysisto eventually cause plugging.

It has now been discovered that the formation of polymer accompanyingthe pyrolysis of tetrafluoroethylene to higher molecular weightperfluorocarbon compounds can be suppressed by intimately contacting thepyrolyzate with an effective polymer suppressing amount of gaseous HClor HF, or a solution of fluoride ion or HCl, before the pyrolyzate coolsto a temperature at which polymerization occurs, which is believed to beat about 500 C.

In one embodiment, the gaseous HCl or HF is injected into the gasesexiting the pyrolysis furnace (the pyrolyrate) to obtain intimatecontact therewith. The amount of HCl or HF so injected will generally bein amounts from 3 to 10 percent of the weight of the tetrafluoroethylenefeed to the pyrolysis furnace.

In another embodiment, the gaseous HCl or HF is supplied by copyrolysisof compounds which convert thereto, with the tetrafluoroethylene feed tothe pyrolysis furnace. Exemplary of compounds which convert to HCl andHF under tetrafluoroethylene pyrolysis conditions arechlorodifluoromethane and fiuoroform, respectively. This in situformation of the HCl or HF provides these compounds in intimate contactwith the pyrolyzate as it exits the pyrolysis furnace. The amount ofcompound used to copyrolyze with the tetrafluoroethylene to form the HClor HP will generally be that amount under the pyrolysis conditionsemployed to supply the 3 to 10 percent range of HCl or HP contentspreviously described herein.

This copyrolysis can be conducted by several methods. In one method, thecompound convertible to either HCl or HP is fed to the pyrolysis furnacewith the tetrafluoroethylene feed. Generally, no greater than 35 percentby weight of the convertible compound, particularly in the case ofchlorodifluoromethane, based on the total weight of the feed to thefurnace is used under this method, because at higher amounts the overallyield to useful products decreases significantly. Usually, at least 20percent by weight of the convertible compound, based on the total weightof the feed, is required to get noticeable improvement in polymersuppression.

In another copyrolysis method, the compound convertible to HCl or HF isinjected into the pyrolysis furnace, slightly upstream of the end of thepyrolysis Zone therein, so that some pyrolysis to HCI or HF occursbefore leaving the furnace. Equipment suitable for this injection isshown, e.g. in FIG. 1 of US. Pat. No. 2,979,- 539 to Errede et al. Inthis copyrolysis method, less of the injected compound will be convertedto the desired HCl or HP than in the previous method because of theshorter exposure to pyrolysis conditions. Consequently, the use ofgreater proportions of the convertible compound will be necessary. Thelower conversion generally accompanying this method, however, has theadvantage of producing lesser amounts of undesirable by-products. I

These copyrolysis methods can be used in combination with one another,or the copyrolysis method of feeding the convertible compound with thetetrafluoroethylene feed into the furnace can be used in combinationwith intimate contacting by HCl or HF added to the pyrolyzate. In thesecombinations of embodiments, the feed of convertible compound orinjection thereof or addition of HCl or HF to the pyrolyzate areproportiond to give the 3 to 10 percent range of HC] or HF in thepyrolyzate prior to its cooling to temperatures whereat polymerizationoccurs. Generally, at least 20 percent by weight of the total HCl or HPpresent in the pyrolyzate will be derived from each source thereof whenthese combinations of embodiments are employed.

In another embodiment of the present invention, the pyrolyzate from thepyrolysis of tetrafluoroethylene is bubbled through or otherwisescrubbed with an aqueous solution of HCl or fluoride ion suppliedby suchcompounds as HF and the alkali metal salts, e.g., KF and NaF. Thepyrolyzate is above temperatures at which the polymerization occurs whenbrought into contact with the aqueous solution which quenches thepyrolyzate. The concentration of the solution and its contact time withthe pyrolyzate can be widely varied to suppress the formation ofpolymer.

The improvement obtained by the present invention is generallyapplicable to the pyrolysis of tetrafluoroethylene to useful highermolecular weight perfluorocarbon compounds, which are mainlyhexafiuoropropylcne and octafluorocyclobutane. Thus, the pyrolysis canbe conducted at both sub-atmospheric and at super-atmospheric pressures,such as from 25 mm. Hg to 65 p.s.i. Temperatures of from 600 to 1000 C.can be used. The feed rate of the tetrafluoroethylene through thepyrolysis furnace is adjusted to give the exposure time required at theparticular furnace operating conditions employed to give the conversiondesired, which will generally be between 30 and 90 percent. Theseparation of unreacted constituents from the pyrolyzate andpurification of the reaction products therein are done by conventionalprocedures, such as disclosed in US. Pat. No. 2,406,794 to Benning etal.; US. Pat. No. 3,009,966 to Hauptschein et a1.; and US. Pat. No.3,101,304 to Wiist.

Examples of the present invention are as follows; parts and percents areby weight unless otherwise indicated.

EXAMPLE 1 Tetrafluoroethylene (TFE) was fed at one atmosphere pressureand ml./sec. STP to a 0.7 cm. diameter, 28 cm. long Inconel pyrolysistube heated by an electric furnace to 790 C. The exit gases were cooledto room temperature and analyzed by gas chromatography. Polymer wascollected in the cold lines at the exit of the reaction tube. Theresults were as follows: TEE conversion=75.59 percent; yield tohexailuoropropylene (HFP) and octafiuorocyclobutane (dimer) =88.3percent; amount of polymer collected in three hours=0.454 g.; totalamount TFE fed=241 g.; amount of TFE polymerized:0.18.8 weight percent.

This experiment was repeated, except that the feed to the tube consistedof 4 ml./sec., the pyrolysis temperature was 780 C. andchlorodifluoromethane was at 25 C. injected at l ml./sec. into the exitgases at the exit end of the pyrolysis tube, with the following results:TFE conversion=75.7 percent; chlorodifluoromethane conversion=13.4percent; yield to HFP and dimer==92.2 percent. Total amount of TFE fedin 3 hours and 25 minutes: 219 g. No polymer collected in exit lines.

EXAMPLE 2 7.9 ml./sec. TFE and 2.25 ml./sec. chlorodifluoromethane werepremixed at room temperatures and fed to the pyrolysis tube described inExample 1, at a temperature of 692 C. and at one atmosphere pressure,with the following results: TFE conversion=35 .8 percent;chlorodifluoromethane conversion=6l.8 percent; yield to HFP anddimer=93.8 percent (3 percent HFP); polymer collected in the exitlines=0.235 g. The total amount of TFE fed over one hour and 45minutes=272.2 g.; TFE polymerized: .086 weight percent.

EXAMPLE 3 6 mL/sec. TFE plus 2.35 mL/sec. chlorodifluoromethane werepremixed and fed to the pyrolysis tube described in Example 1, at atemperature of 795 C. and a pressure of one atmosphere, with thefollowing results: TFE conversion=7 3.3 percent; chlorodifiuoromethaneconversion=94.0 percent; yield to HFP and dimer=693 percent. The totalamount of TFE fed over 2 hours and 30 minutes=241 g.

Less than one milligram polymer was detected in the exit lines, whichcorresponds to 0.004 percent of the TFE fed being polymerized.

EXAMPLE 4 3.8 ml./sec. TFE plus 1.7 ml./ sec. chlorodifiuoromethane werepremixed and pyrolyzed at 763 C. in the pyrolysis tube described inExample 1, at one atmosphere pressure. 0.9 rnl./sec.chlorodifluoromethane at 25 C.

4 were injected at the exit. The injection tube extended 5 cm. into the28 cm. long pyrolysis tube. The following results are obtained: TFEconversion=70.7 percent; chlorodifiuoromethane conversion=61.8 percent;yield to HFP and dimer=8l.8 percent. The total amount of TFE fed overthree hours=183.3 g. No polymer was detected in exit lines.

EXAMPLE 5 1.7 mL/sec. chlorodifiuoromethane and 8 ml./sec. TFE werepremixed at room temperature and fed to the pyrolysis tube described inExample 1. The tube was heated to 690 C. 3.3 ml./sec.chlorodiiluoromethane of room temperature were injected at the tubeexit. The following results were obtained: TFE conversion:39.8 percent;chlorodifluoromethane conversion=29.6 percent; yield to HFP anddimer=94.3 percent. Total amount of TFE fed in 2 hours and 35minutes=332 g.; polymer formed '=0.0426 gram; TFE polymerized=0.0128weight percent.

EXAMPLE 6 2.2 mL/sec. of TFE were fed to the pyrolysis tube described inExample 1. The tube was heated to 658 C. and was at one atmospherepressure. The exit gases were immediately scrubbed by bubbling through a0.05 percent solution of sodium fluoride in water at room temperature.The following results were obtained: TFE conversion=85.l percent; yieldto HFP and dimer=9'5.4 percent; total amount of TFE fed=94.4 g. Nopolymer in lines downstream of scrubber. Similar results were obtainedwhen HCl, HF and KF were individually substituted for the NaF.

EXAMPLE 7 5 ml./sec. TFE plus 1.8 ml./sec. chlorodifluoromethane weremixed and fed to the pyrolysis tube described in Example 1. The pressurewas reduced to 0.35 atmosphere and the tube heated to 863 C., with thefollowing results being obtained: TFE conversion=52.-7 percent;chlorodifluoromethane=9L8 percent; yield to HFP and dimer =8l.2 percent.

395 g. TFE were fed over 4 hours and 55 minutes. Less than 10 miligramspolymer deposited on Walls in the exit lines. This is 0.0025 weightpercent of total TFE fed.

The conversions and yields in the foregoing examples were calculatedusing the following equations wherein only the carbon and fluorinecontent of each component in the feed and pyrolyzate is calculated, tothe exclusion of hydrogen and chlorine:

Percent chlorodifluoromethane converted: CF wt. percent(ehlorodifluoromethane in feedehlorodifluoromethane in pyrolyzate) X CFwt. percent chlorodifiuoromethaue in feed Percent TFE converted= CF wt.percent (TFE in feed- TFE in pyrolyzate) X 100 CF wt. percent TFE infeed from the exit end of said furnace, and intimately contacting thepyrolyzate at a temperature above about 500 C. with between about 3% andabout 10% by weight, based upon the weight of tetrafluoroethylene fedinto said furnace, of gaseous HCl or HP or bubbling through or otherwisescrubbing said pyrolyzate with an aqueous solution of HCl, HF or alkalimetal fluoride salts.

2. The process of claim 1 wherein said 'HCl is formed by copyrolyzingchlorodifluoromethane with said tetrafluoroethylene.

3. The process of claim 2 wherein said chlorodifluoromethane is suppliedwith the tetrafluoroethylene feed to said furnace.

4. The process of claim 2 wherein said chlorodifluoromethane is injectedinto the exit end of the pyrolysis zone of said furnace.

5. The process of claim 2 wherein a portion of saidchlorodifluoromethane is supplied with the tetrafluoroethylene feed tosaid furnace and the remainder thereof 6 is supplied by injection intothe exit end of the pyrolysis zone of said furnace.

6. The process of claim 1 wherein the compound used for polymersuppression is HCl in gaseous form and a portion thereof is supplied bycopyrolysis 0f chlorodifluoromethane supplied with thetetrafluoroethylene feed to said furnace.

References Cited UNITED STATES PATENTS 2,979,539 4/1961 Errede et al260-6533 3,459,818 8/1969 Ukihashi et al 260-6533 3,308,174 3/1967Edwards et a1 260-6535 FOREIGN PATENTS 1,236,497 3/1967 Germany 260-6533DANIEL D. HORWITZ, Primary Examiner US. Cl. X.R. 260-6533

